Conventional electromagnetic propagation based tools for measurement of formation resistivity while drilling typically include a cylindrical tool body, which forms a portion of the drillstring, and at least one transmit antenna and a pair of receive antennas mounted within the tool body. The antennas are typically electrostatically shielded wire loops that circumferentially surround the tool body and are mounted in circumferential grooves defined in the surface of the tool body. The tool employs magnetic dipoles operating in the MF and HF regions. The transmit antenna generates electromagnetic waves which are propagated through the formation surrounding the borehole. The waves are detected as they pass the respective receive antennas. The signal detected at the further of the two receive antennas is shifted in phase from and lower in amplitude than the signal detected at the closer of the two receive antennas. Formation resistivities are derived from the phase difference and amplitude ratio of the signals detected. The differential measurement is primarily indicative of the properties of the region of the formation surrounding the receivers and is less sensitive to other regions of the formation and to variations in the transmitted signal.
A shortcoming of conventional measurement-while-drilling (MWD) formation evaluation tools is low efficiency of the antennas due to dominance of the electromagnetic field by the tool body.